Neural event detection

ABSTRACT

A method of detecting an induced muscle response includes monitoring a mechanical parameter of a muscle; computing an amount of muscle jerk from the monitored parameter; comparing the computed muscle jerk to a jerk threshold; and identifying the occurrence of an induced muscle response if the computed muscle jerk exceeds the threshold.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of and claims the benefit ofpriority from U.S. application Ser. No. 12/818,319, filed Jun. 18, 2010(“the '319 application”), which is a continuation-in-part and claims thebenefit of priority of U.S. application Ser. No. 12/605,020, filed Oct.23, 2009 (“the '020 application”), which is a continuation-in-part andclaims the benefit of priority of U.S. application Ser. No. 12/040,515(“the '515 application”), filed Feb. 29, 2008, which claims the benefitof priority to U.S. Provisional Application No. 60/980,996 (“the '996application”), filed Oct. 18, 2007. The '020 application further claimsthe benefit of priority from U.S. Provisional Application Nos.61/108,214 (“the '214 application”), filed Oct. 24, 2008 and 61/229,530(“the '530 application”), filed Jul. 29, 2009. The entire disclosures ofthe '319 application, '020 application, the '515 application, the '996application, the '214 application, and the '530 application are herebyincorporated by reference as though fully set forth herein.

BACKGROUND

The present disclosure relates generally to a neural monitoring devicethat may be capable of detecting the proximity of a nerve from aninvasive stimulator, and monitoring for potential nerve injury during asurgical procedure. Traditional surgical practices emphasize theimportance of recognizing or verifying the location of nerves to avoidinjuring them. Advances in surgical techniques include development oftechniques including ever smaller exposures, such as minimally invasivesurgical procedures, and the insertion of ever more complex medicaldevices. With these advances in surgical techniques, there is acorresponding need for improvements in methods of detecting and/oravoiding nerves.

SUMMARY

A method of detecting an induced muscle response includes monitoring amechanical parameter of a muscle, such as, for example, an accelerationof the muscle; computing an amount of muscle jerk from the monitoredparameter; comparing the computed muscle jerk to a jerk threshold; andidentifying the occurrence of an induced muscle response if the computedmuscle jerk exceeds the threshold. In an embodiment, the jerk thresholdmay be an increasing function of the sensed peak acceleration.

In an embodiment, the method further includes comparing the sensedacceleration to an acceleration threshold, and providing an audible orvisual alert if an induced muscle response is detected and the sensedacceleration exceeds the acceleration threshold. In an embodiment, theacceleration threshold may include both a recurring event threshold anda singular event threshold, where the recurring event threshold is lowerthan the singular event threshold.

In an embodiment, the method further includes receiving an indication ofthe amplitude of an applied electrical stimulus from an invasivestimulator and using the applied stimulus along with the sensedacceleration to determining a distance between the invasive stimulatorand an innervated nerve.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary embodiment of a neural monitoringsystem.

FIG. 2 is a schematic illustration of an embodiment of a neuralmonitoring system and a treatment area of a human subject.

FIG. 3 is an illustration of a stimulator probe within a treatment areaof a subject.

FIG. 4 is an illustration of an exemplary placement of a plurality ofsensing devices.

FIGS. 5A-5F are illustrations of various embodiments of a sensingdevice.

FIG. 6 is a schematic diagram of an embodiment of a sensing device.

FIG. 7 is a schematic diagram of an embodiment of a sensing device.

FIG. 8 is a schematic diagram of an embodiment of a receiver.

FIG. 9 is a graph of a electromyography response to an applied stimulus.

FIG. 10 is a flow chart illustrating an exemplary muscle responsedetection scheme.

FIG. 11A is a graph illustrating an exemplary jerk threshold.

FIG. 11B is a graph illustrating an exemplary muscle response.

FIG. 12A is a graph illustrating an exemplary correlation betweenstimulator current, measured muscle response, and stimulator proximityto a nerve.

FIG. 12B is the graph of FIG. 12A including a desired threshold.

FIG. 13A is an illustration of an embodiment of a stimulator.

FIG. 13B is an enlarged view of the stimulator of FIG. 13A.

FIG. 14 is an illustration of an exemplary embodiment of a stimulatorincorporated with an invasive medical device.

FIG. 15 is a schematic illustration of an embodiment of a neuralmonitoring system including a transdermal stimulator.

DETAILED DESCRIPTION

Referring to the drawings, wherein like reference numerals are used toidentify like or identical components in the various views, FIG. 1illustrates an exemplary neural monitoring system 10 that includes areceiver 12 in communication with a plurality of sensing devices 14, astimulator 16, and a ground patch 18. In an embodiment, the receiver 18may include an interface 20 and a computing device 22. The computingdevice 22 may include a processor, memory, and a display, such as forexample, a personal computer, tablet computer, personal digitalassistant (PDA), or the like. The interface 20 may be configured toreceive and present information from the one or more sensing devices 14to the computing device 22, and may include, for example, communicationscircuitry, signal processing circuitry, and/or other associatedinterfacing circuitry. While shown as distinct components in FIG. 1, inan embodiment, the interface 20 may be an integral part of the computingdevice 22.

FIG. 2 schematically illustrates an embodiment of a neural monitoringsystem 10 being used with a human subject 30. As shown, the neuralmonitoring system 10 includes a receiver 12, a stimulator 16, and asensing device 32. The stimulator 16 may be configured to provide astimulus 34 within a treatment region 36 of the subject 30. Exemplarytreatment regions 36 may include the posterior, posterolateral, lateral,anterolateral or anterior regions of the sacral, lumbar, thoracic orcervical spine, as well as the tissue surrounding such regions. Thestimulator 16 may be configured to provide the stimulus 34 constantlyduring a surgical procedure, or selectively at the discretion of thesurgeon.

As shown in FIG. 3, in an embodiment, the stimulator 16 may include aprobe 38 or other invasive medical instrument configured to extendwithin the treatment region 36 of the subject 30, and provide a stimulus34 therein. The stimulus 34 may be, for example, an electrical stimulus,though may alternatively be a thermal, chemical, ultrasonic, or infraredstimulus, or may include a direct mechanical contact with the nerve. Ifthe stimulus 34 is provided at or sufficiently close to a nerve withinthe treatment region 36 (e.g., nerve 40), the stimulus 34 may bereceived by the nerve in a manner that causes the nerve to depolarize. Adepolarizing nerve may then induce a response in a muscle that isinnervated by the nerve. Exemplary muscle responses may include, forexample, physical motion, acceleration, displacement, or vibration ofthe muscle, and/or changes in muscle's electrical polarity. While FIGS.2 and 3 illustrate the treatment region 36 including the lumbar spine,it is understood that the present invention may be used in connectionwith other surgical or therapeutic procedures that may be performed inthe proximity of other peripheral motor nerves.

As generally illustrated in FIG. 2, the neural monitoring system 10 mayinclude one or more sensing devices 32 that are configured to detectmechanical and/or electrical responses of various muscles of the subject30. In an embodiment, a sensing device 32 may be affixed to the skin ofthe subject 30 in a manner that places it in communication with aparticular muscle or muscle group innervated by a nerve within thetreatment area 36. For example, as shown, the sensing device 32 may beplaced in communication with a quadriceps muscle 42 of the subject 30.As used herein, the sensing device may be considered to be incommunication with a muscle if it is sufficiently proximate to themuscle group to sense a mechanical and/or electrical parameter of themuscle. A sensed mechanical parameter may include, for example, musclemotion, acceleration, displacement, vibration, or the like Likewise, asensed electrical parameter may include an electrical potential of themuscle, such as when the innervated muscle is electrically orelectrochemically activated.

By way of example, and not limitation, during a discectomy of the lumbarspine, a surgeon may know that the nerves exiting the L2, L3 and L4foramen are potentially located in the treatment region 36. Asillustrated in FIG. 4, the surgeon may place a sensing device 32 on eachmuscle innervated by those nerves. For instance, sensor devices 44, 46may be placed on the vastus medialis muscles, which are innervated bynerves exiting the L2 and L3 foramen. Likewise sensors 48, 50 may beplaced on the tibialis anterior muscles, which are innervated by thenerves exiting the L4 foramen. If a muscle response is then detected byone of these sensor devices, the surgeon may then be alertedaccordingly.

FIGS. 5A-5F illustrate various embodiments of a sensing device 32. Asshown in FIG. 5A, the sensing device 32 may be affixed to the skin 52 ofthe subject 30 in such a manner that it is in mechanical and/orelectrical communication with a particular muscle or muscle group of thesubject (e.g., quadriceps muscle 42 as shown in FIG. 2). In anembodiment, the sensor device 32 may include a cable 54 configured toconnect with an interface 20 of a receiver 12, an adhesive patch portion56 that may adhere the sensor device to the skin 52 of the subject 30,and an instrument portion 58. As generally illustrated in FIG. 5B, theinstrument portion 58 may include a circuit board 60 and one or moreelectrical components 62. In an embodiment, the circuit board 60 may bea rigid circuit board, such as one made from, for example, an FR-4substrate. Alternatively, the circuit board 60 may be a flexible circuitboard, such as one made from a polyimide, PEEK, polyester, or otherflexible substrate. In an embodiment, as shown in FIG. 5A, theinstrument portion 58 of the sensor device 32 may be enclosed by aprotective cover 64 that may serve as a fluid barrier and protect theinternal electrical components 62 from external moisture.

As illustrated in FIGS. 5B-5C, in an embodiment, the sensor device mayhave two or more surface electrodes 66, 68 and/or needle electrodes 70,72 that are configured to be placed in electrical communication with theskin and/or muscle of the subject 30. In an embodiment, the surfaceelectrodes 66, 68 may be configured to make electrical contact with theskin 52 of the subject 30 to monitor the electrical parameters of theadjacent muscle (e.g., quadriceps muscle 42) and/or to detect contactwith the subject. Surface electrodes may require the surface of the skinto be shaved or coated with an electrically conducting gel to improvethe electrical connectivity with the skin 52. Conversely, needleelectrodes may penetrate the skin and extend directly into the musclebelow.

As illustrated in FIG. 5B, the electrodes, such as needle electrodes 70,72, may be integrated into the sensing device 32 in a fixed locationand/or arrangement. Through the fixed attachment with the circuit board60, each electrode 70, 72 may provide a respective electrical signal tothe one or more electrical components 62 via the circuit board 60. Asillustrated in FIG. 5C, in an embodiment, the sensor device 32 may beconfigured to accept removable needle electrodes 71, 73 that may passthrough respective apertures 74, 76 in the circuit board 60, and maycouple to the one or more electrical components 62 via respectivebrushes, contacts, slip rings, wires 78, 80 or other known electricalcontact means.

FIG. 5D illustrates another embodiment of a sensing device that includesa central instrument portion 58 and two adjacent adhesive portions 82,84. The instrument portion 58 may include one or more electricalcomponents 62 affixed to a circuit board 60, and each adhesive portion82, 84 may include a respective adhesive patch 56, and/or one or moresurface or needle electrodes. In an embodiment, each adhesive portion82, 84 may include a respective aperture 74, 76 configured to receive aneedle electrode (e.g., needle electrodes 71, 73). Additionally, in anembodiment, each adhesive portion 82, 84 may include an electricallyconductive pad 86, 88 surrounding respective apertures 74, 76 that maybe configured to make electrical contact with a needle electrode passingthrough the respective apertures.

FIGS. 5E and 5F illustrate two further embodiments of a sensing device32. In each embodiment, the sensing device 32 includes one or moreelectrical components 62 that are configured to sense one or moreparameters of a muscle of a subject. In an embodiment, the electricalcomponents 62 may include a mechanical sensor configured to detectand/or provide a signal corresponding to a mechanical movement of amuscle. An exemplary mechanical sensor may include an accelerometerdesigned to monitor motion in one or more axes. The one or moreelectrical components 62 may additionally be adapted to interface with aplurality of electrodes for the purpose of monitoring an electricalparameter of a muscle and/or detecting contact with the subject.Exemplary electrode configurations are illustrated in FIGS. 5E and 5F,(i.e., surface electrodes 90 a, 90 b, and surface electrodes 92 a, 92 b,92 c, 92 d). As described above, the design of the sensing device 32 maybe altered to accommodate needle electrodes in addition to, or insteadof the surface electrodes.

In an embodiment where the sensing device 32 includes both a mechanicalsensor and a plurality of electrodes, it may be beneficial to locate themechanical sensor as close to the center of the device as possible.While not strictly necessary, such a configuration, as generallyillustrated in FIGS. 5D-5F, may allow the greatest amount of adhesivematerial 56 to surround the mechanical sensor and thus improve itsmechanical coupling with the skin.

The sensing device 32 may further be configured for stand-alone use, asgenerally shown in FIGS. 5E and 5F. In an embodiment, the sensing device32 may include a local receiver module 94 that may receive the signalsfrom the mechanical and/or electrical sensors and detect when a muscleevent occurs. Additionally, the local receiver module 94 may beconfigured to provide an alert indication if such a muscle event isdetected. In an embodiment, the indication may be provided byilluminating an associated light emitting diode (LED) 96, oralternatively by changing the color of an LED, such as from green tored. In another embodiment, the receiver module 94 may emit a sound thatis indicative of a muscle movement. The receiver module 94 may beincluded with each sensor device 32 either by integrating it with theone or more electrical components 62, or by providing it as a detachabledevice similar to the module 94 shown in FIGS. 5E and 5F. The localreceiver module 94 may further include a power source, such as a batteryto provide power to the various electrical components.

In an embodiment, the local receiver module 94 may include all of thefunctionality and event detection capabilities of a more centralizedreceiver (such as the receiver 12 illustrated in FIG. 1). In acoordinated system that employs multiple sensors, each local receiver 94may be configured to communicate alerts with a master receiver 12 usingwired or wireless data communication means. In an embodiment, the masterreceiver 12 may aggregate the occurrence and/or timing of local eventsinto a consolidated interface.

FIGS. 6 and 7 illustrate electrical diagrams of various embodiments of asensor device 32. These diagrams may generally represent the one or moreelectrical components 62 that are included with the device. In anembodiment, the sensor device 32 may include a mechanical sensor 100,and an electrical sensor 102. Each sensor may be configured to provide arespective output signal 104, 106 that may correspond to a parametermonitored by the sensor. Each output signal 104, 106 may be configuredfor either wired or wireless transmission to the receiver 12. In anembodiment, each output signal 104, 106 may include a respective voltagethat corresponds to the monitored parameter. Alternatively, each outputsignal may include a variable current or a variable resistance signalthat corresponds to the monitored parameter. For example, the outputsignal 104 from the mechanical sensor 100 may be a mechanomyographyvoltage signal (V_(MMG)), and the output signal 106 from the electricalsensor 102 may be an electromyography voltage signal (V_(EMG)). Eachsensor 102, 104 may be configured to monitor for both triggered muscleresponses (i.e., muscle responses that occur in response to astimulator-applied stimulus 34) and for free-running muscle responses(i.e., muscle responses that may occur in the absence of astimulator-applied stimulus 34).

In an embodiment, the mechanical sensor 100 may be configured to detecta mechanical response of the muscle or group of muscles that are incommunication with the sensing device 32. The mechanical response mayinclude, for example, muscle motion, acceleration, displacement,vibration, etc. In one exemplary approach, the mechanical sensor 100 maybe an accelerometer configured to detect acceleration in at least oneaxis (e.g., in the direction normal to the surface of the skin, asrepresented by the z-axis in FIG. 5A). In an embodiment, the outputsignal 104 of the mechanical sensor 100 may be a voltage thatcorresponds to the sensed movement. The output signal 104 may indicateone or more directions, axes, and/or magnitudes, of motion,acceleration, displacement, or vibration experienced by mechanicalsensor 100. In an embodiment, mechanical sensor 100 may be accelerometermodel MMA7361 available from Freescale Semiconductor.

The electrical sensor 102 may be configured to detect an electricalresponse of the muscle or group of muscles that are in communicationwith the sensing device 32. The electrical sensor 102 may include aplurality of electrodes that are configured to be placed incommunication with the muscle of the subject 30, either through thesurface of the skin, or by extending through the skin and making directcontact with the muscle itself. The plurality of electrodes may includea first, “positive” electrode 108, and a second, “negative” electrode110. Additionally, in an embodiment, the electrical sensor may include areference electrode 112. The positive and negative electrodes 108, 110may each monitor a polarity of a portion of the muscle that it is incommunication with. The monitored polarity may be viewed with respect toa common reference electrode, such as electrode 112, which may beincluded with the sensing device 32 or may be separate from the device.In an embodiment, one single reference electrode may be used for aplurality of sensing devices, and may be included with the system as adistinct patch electrode, such as ground patch 18, illustrated in FIG.1.

As illustrated in FIG. 6, in an embodiment, each electrode 108, 110, 112of the electrical sensor 102 may pass an unfiltered, unamplified outputsignal directly to the receiver 12. In another embodiment, such asillustrated in FIG. 7, each electrode may first connect to a localamplification or isolation circuit 114. As illustrated, theamplification circuit 114 may compare the potentials monitored by eachof the positive and negative electrodes 108, 110 with the potentialmonitored by a local reference electrode 112 using respectivecomparators 116, 118. These normalized signals may then be compared toeach other through a third comparator 120, and the resulting output maybe provided to the receiver 12 as a single output signal 106.Alternatively, if no local reference electrode exists, comparators 116and 118 may be omitted and the positive and negative electrodes 108, 110may feed directly into comparator 120. Comparator 120 may further beconfigured to amplify or boost the output signal 106 for transmissionback to the receiver.

In an embodiment, as shown in FIG. 6, the sensing device 32 may furtherinclude a contact detection device, such as a power circuit 130configured to monitor one or more electrodes (e.g., electrodes 108,110), and energize the mechanical sensor 100 when contact with thesubject 30 is detected. In an embodiment, as shown in FIG. 7, the powercircuit 130 may also energize an amplification or isolation circuit 114of the electrical sensor 102, if such a circuit is provided.

The power circuit 130 may, for example, include a capacitive switch thatselectively provides power when a capacitance between the electrodes isat or below a certain threshold. Alternatively, the power circuit 130may energize the sensor components when a threshold background orbaseline electric field is detected. Alternatively, the power circuit130 may energize the sensor components when a threshold background orbaseline electrical signal is detected. The presence of such abackground electrical activity (such as free-running EMG activity) mayindicate that the sensor is in contact with the subject, as it does notexist apart from the subject. If such electrical activity is detected,the power circuit may act as a high impedance relay and provide power tothe various components.

In an embodiment, the power circuit 130 may create an alert condition ifcontact with the subject 30 is lost. The alert condition may include thetransmission (or lack thereof) of a separate contact signal to thereceiver 12, or may include the absence of a mechanical output signal.For example, if the electrodes become decoupled from the subject 30, thebaseline electrical activity or impedance sensed by the power circuitmay disappear. Upon this drop-out, the power circuit 130 may switch offthe supply power to the mechanical sensor 100 and cause the sensor 100to stop transmitting a mechanical output signal 104. The receiver 12 mayinterpret the break in transmission as a loss of sensor contact, whichmay be conveyed to the user through an appropriate alert.

As described above, the sensing device 32 may provide an output signal(e.g. mechanical output signal 104 and/or electrical output signal 106)to a receiver 12 for processing. FIG. 8 illustrates a schematicrepresentation of the receiver 12, which may be similar in function to alocal receiver module 94. In an embodiment, the mechanical and/orelectrical output signals 104, 106 may each pass through a respectivesignal conditioning circuit 200, 202, which may amplify the signaland/or filter out any unwanted noise. The filtered signals may then bereceived by an event processor 206 where they may be analyzed todetermine their relationship to an applied stimulus 34. Additionally,the event processor 206 may be in communication with the stimulator 16through a stimulus signal 208 for the purpose of correlating a detectedevent with an applied stimulus 34. The receiver 12 may further include adisplay processor 210 that is configured to provide graphical feedbackto the user.

In an embodiment, the signal conditioning circuitry 202, 204 may includea band-pass filter that may filter out the DC component of the signals,along with any unwanted higher frequency components. In an exemplaryembodiment, and without limitation, the filter may have a high-passcutoff frequency in the range of 0.1-0.5 Hz, and may have a low-passcutoff frequency in the range of 75-125 Hz.

The event processor 206 may analyze the filtered signals to, forexample, detect the occurrence of an electrical event 220, detect theoccurrence of a mechanical event 222, determine if a detected eventcorresponds to an applied stimulus 224, determine the proximity of anerve from an applied stimulus 226, determine if a sensor has becomedisconnected from the subject 228, and/or determine if the surgeonshould be provided with an alert 230.

In an embodiment, as shown in FIG. 9, and exemplary electrical responseto an applied pulse stimulus may include three components: a stimulusartefact 250, a muscle motor response 252 (also referred to as the“M-Wave”), and the Hoffmann Reflex 254 (“H-Reflex”). The stimulusartefact 250 may be a direct result of the applied electrical currentwithin the body, and may not reflect a nerve's ability to transmit anaction potential. Quite to the contrary, the M-Wave 252 is the actionpotential within a muscle that is caused by the depolarization of anerve. This action potential is the primary cause of a naturalmechanical motor response of a muscle, and is a result of theelectrochemical activity of the motor neurons. Similar to the M-Wave252, the H-Reflex 254 is a nerve-transmitted reflex response that mayprovide useful information about the presence or function of a nervelocated proximate to the stimulator. In an embodiment, the receiver 12may analyze the electrical output signal 106 to detect an M-Wave 252 orH-Reflex 254 electrical event. The system may then compare the magnitudeof the detected electrical event with a pre-determined threshold toprovide a general indication of proximity between the stimulator and agiven nerve.

In practice, traditional systems may have difficulty differentiating theM-Wave 252 from the stimulus artefact 250 due to the duration andmagnitude of the artefact and the close timing of the two events. Tocreate a more robust detection system, the receiver 12 may analyze themechanical sensor output 104 for the existence of mechanical events 222and/or attempt to correlate the mechanical events with the electricalevents. Because mechanical events are generally not susceptible to thestimulus artefact 250, they may be used to enhance the sensitivityand/or specificity of a purely electrical detection system.

In an exemplary embodiment, mechanical sensor 100 may comprise anaccelerometer. As illustrated in FIG. 10, the receiver 12 may detect theexistence of mechanical events 222 and/or correlate the events to anapplied stimulus 224 by first registering raw readings from theaccelerometer in step 300 (e.g., mechanical output signal 104). Thesystem may then use these raw readings to derive the amount of muscle“jerk” experienced by the patient (“jerk,” or a “jerk value,” is therate of change of the sensed acceleration (i.e. da/dt)). While a jerkvalue may be derived by taking the time derivative of acceleration, itmay also be computed from other sensed mechanical parameters, such asvelocity or position. It has been found that a muscle response inducedby a provided stimulus may correspond to a particular jerk rate. Bysetting an appropriate threshold and comparing the derived jerk to thethreshold (step 302), the system may be able to initially filterrecorded readings to discriminate between a stimulator induced response,a patient-intended muscle movement, and an unintended environmentalresponse (e.g. bumping the patient table). Finally, by comparing theamplitude of the sensed acceleration to a threshold (step 304), thesystem may determine whether the innervated nerve is sufficiently closeto the stimulator to alert the physician. It should be understood thatthe jerk evaluation (step 302) may occur either before or after testingthe amplitude of the sensed acceleration (step 304) without affectingthe spirit of the invention.

Jerk and/or acceleration thresholds may be separately provided for eachsensor at the discretion of the physician. In an embodiment where alocal receiver 94 is included with each sensor device 32, such asillustrated in FIGS. 5E and 5F, the thresholds may be modified from acentral control system, such as receiver 12, and remotely programmedinto each device. In such an embodiment, local event detection mayoperate by monitoring the mechanical and/or electrical response of theproximate muscle according to the associated thresholds. A muscle twitchalert may comprise a visual or audible indication on the sensor itselfif the individual thresholds are crossed and a muscle event is detected.

In an embodiment incorporating electrical stimulation, the system mayfurther detect whether an electrical stimulus was transmittedimmediately prior to a sensed response. This correlation may allow thesystem to further relate a sensed muscle response to the physician'sactions. The system may use the stimulus correlation to alert thephysician of a potentially applied manual stimulus (i.e., if a muscleresponse was detected in the absence of an electrical stimulus, theresponse may indicate a physical contact with, or manipulation of thenerve that innervates the responding muscle). In other embodiments,other sensed or derived parameters may be used for the purpose ofidentifying stimulator-induced muscle response, as well as for testingthe magnitude of the induced response.

The thresholds used in steps 302 and 304 for detecting an event may bevaried based on the type or timing of the detected sensor response. Forexample, in an embodiment, as generally shown in FIG. 11A, the jerkthreshold 310 may be an increasing function of sensed accelerometer peakamplitude (in mV) In an embodiment, as generally illustrated in FIG.11B, when analyzing an accelerometer output 312, a higher accelerationthreshold 314 may be used for detecting a singular event (e.g., event316), while a lower threshold 318 may be used for recurring events(e.g., events 320, 322, 324). Likewise, the system may use a loweracceleration threshold for events occurring within a specified timeperiod following the application of a stimulus.

The above described system may be used to aid a physician in avoidingcontact with a nerve. As described above, this may be accomplished byalerting the physician when he/she brings the stimulator within acertain proximity of a nerve. In another embodiment, the above describedsystem may be used to aid a physician in locating a particular nerve,such as during a pain management procedure. As known in the art, certainpain management procedures require injecting a local anesthetic at, orin proximity of, a sensory nerve. By locating the motor nerve throughthe proximity detection methods described above, the physician may moreaccurately identify an injection site for the anesthetic.

To further aid in neural proximity detection the receiver 12 may beconfigured to determine the proximity of a nerve from an appliedstimulus 226 based on the electrical current of the applied stimulus andthe measured mechanical sensor signal output. As generally shown inFIGS. 12 a and 12 b, correlation graphs may be used to provide thesystem or physician with an idea of the absolute proximity of thestimulator to the nerve. Correlation graphs, such as those shown in FIG.12 a, may be empirically determined on a patient-by-patient basis, ormay be theoretically derived based on factors such as the thickness anddensity of the patient's skin, subcutaneous fat, and muscle.Alternatively, general correlation graphs such as illustrated in FIG. 12a may be generated, and provided with confidence bands or modified tosuit a particular patient based on factors specific to the patient (e.g.body mass index).

In an exemplary approach, a physician may dictate the current level thatis being applied to the stimulator, if the stimulator is close enough toa nerve to induce a muscle response, the sensing device 32 (such asillustrated in FIGS. 5-7) would generate an output signal correspondingto measured parameters, which may be quantified by the system. Thesystem may use this knowledge of the stimulus strength and the magnitudeof the mechanical sensor output signal 104 to determine an approximateabsolute distance between the stimulator and the nerve In an embodiment,the system may have a pre-set initial current level that is selectedbased on the intended procedure. For example, when the software startsup the physician may be presented with a screen that inquiring as toeither the type of surgical procedure being performed, or the distanceaway from the nerve the physician wishes to remain. The system may thenuse this information to adjust the threshold based on optimal currentsetting for the procedure or distance. The physician may also maintainthe ability to vary the current level during the procedure.

As generally shown in the correlation graph of FIG. 12 b, a thresholdmay be set within the range of expected sensor signal levels (e.g. asdescribed in connection with FIG. 10 (step 304)). Once a particularsensor signal threshold is set, a physician may then select a staticcurrent based on his/her level of confidence with the procedure. Forexample, as described with reference to FIG. 12 b, if the physician onlywishes to only be alerted when he/she is within 3 mm of a nerve, giventhe pre-set threshold of approximately 1.86 units (e.g., volts), thephysician would conduct the procedure with a 3 mA stimulus current.Alternatively, if the physician only desired to be alerted when within 1mm of a nerve, he/she would conduct the procedure with a 1 mA current.

In an exemplary procedure, a physician may begin by setting a constantsensor threshold, and by setting the stimulator current near an upperend of a range. For example, as shown in FIGS. 12 a and 12 b, such acurrent value may be 6 mA. Using the known stimulus-responsecorrelation, such as illustrated in FIGS. 12 a and 12 b, the system mayprovide an alert when the stimulator is within a particular distance ofthe nerve. In an embodiment, while maintaining the constant threshold,the applied current may be gradually decreased. By gradually dialingdown this current, the physician may further refine his assessment ofthe nerve location. Similarly, the sensor threshold may be adjusted. Forexample, in an application where the physician wants more sensitivity,the threshold can be adjusted lower. Likewise, in an application wherethe physician wants more specificity, the threshold may be adjustedhigher.

As further illustrated in the receiver 32 diagram of FIG. 8, in additionto being able to detect certain electrical and/or mechanical events 220,222, correlate such events to a provided stimulus 224, and use themagnitude of the events to determine a nerve proximity from the appliedstimulus 226, the event processor 206 may be configured to detect when asensing device 32 loses contact with the subject 30. As described above,such a loss of contact may be determined based on a drop-out in themechanical or electrical output signals 104, 106, as would be caused ifa contact-based power circuit 130 ceased providing required power to themechanical and/or electrical sensors 100, 102 (as illustrated, forexample, in FIGS. 6 and 7). Alternatively, the event processor 206 maymonitor the sensing device 32 for the presence of background electricalactivity from the plurality of electrodes (e.g., electrodes 108, 110 inFIGS. 6 and 7). If contact between the electrodes and the subject 30were lost, the background electrical activity (such as free-runningelectromyography activity) would cease, which may be interpreted by theprocessor as the loss of sensor contact. The event processor 206 mayalso be able to differentiate between background electrical activitywhen in contact with a subject and the background electrical activity inopen air.

The event processor 206 may additionally generate alerts 230 that maycorrespond to sensed events, to stimulator proximity within a giventhreshold of a nerve, or to the loss of contact between a sensing device32 and the subject 30. In an embodiment, the alerts may be visual innature, and may be provided to a display processor 210 for display to auser. In an embodiment, the alerts may indicate to the user thelocation, magnitude, and/or nature of a detected event. In anembodiment, the display processor 210 may be integrated with the eventprocessor 206 in a single general purpose processor or PC (for exampleas with computer 22 illustrated in FIG. 1). In an embodiment where eventdetection capabilities are included with the sensor, such as through alocal receiver module 94, the alert generation module 230 may provide avisual and/or audible alert, such as through an on-board light orspeaker, when a muscle event is detected.

During operation, the system 10 may be configured to provide a safe or“GO” signal if all sensing devices 32 are attached to the subject 30,the ground patch 18 is electrically coupled with the subject 30, and nomuscle responses are detected. If the system detects that a sensingdevice 32 or ground patch 18 has lost contact with the subject 30, thesystem may be configured to alert the physician through an audiblealert, or a visual alert such as a stop sign or “NO GO” warning. Suchcontact notification may similarly occur on the sensor itself, such asby illuminating a light with a color that corresponds with a loss ofcontact. In another embodiment, the sensor may provide an audibleindication that it has lost contact with the subject. This warning maybe used to convey that the neural monitoring system 10 isnon-operational. Likewise, the receiver 12 may provide an indication tothe user that may identify which sensor has lost contact. As describedabove, the system may also be configured to alert the physician if theentire system is operational and connected and a muscle response exceedsa threshold.

Therefore, a “GO” signal may represent a fully functioning system wherea nerve is not proximate to the stimulator 16, while appropriatealternate warnings or alerts may further indicate that either the systemis either non-operational and must be re-connected, or that a nerve isin proximity to the stimulator 16.

FIGS. 13A and 13B generally illustrate an embodiment of a stimulator 16,which may be similar to the stimulator 16 illustrated in FIG. 3, andconfigured for intrabody use. Stimulator 16 includes a handle 410, and astimulator probe 38. In an embodiment, the stimulator probe 38 may bedetachable from the stimulator handle 410, and may be replaceable withone or more different types of probes. In an embodiment, stimulatorprobe 38 includes an electrode 430 positioned at the distal end of theprobe that may be configured to deliver a stimulus 34.

The stimulator handle 410 may be connected to an electrical cable 440for transmitting signals between the receiver 12 and the stimulator 16.Handle 410 may include one or more buttons 450, selector devices, wheels460, or LEDs. In an embodiment, a button, such as button 450, may beconfigured to selectively transmit an electrical stimulus 34 throughstimulator probe 420. In an embodiment, rotation of wheel 460 may beconfigured to cycle through options on a display associated with thesystem, and the depression of wheel 460 may be configured to select anoption on such a display. In an embodiment, rotation of wheel 460 may beconfigured to selectively vary the current intensity of the stimulus 34transmitted through probe 38 and electrode 430. Additionally, visualindicators, such as LEDs may be incorporated into handle to conveyinformation to the physician, such as, for example, detection of amuscle response or proximate nerve, a GO/NO-GO indicator, or may simplyprovide feedback to the physician that the stimulator is transmitting anelectrical stimulus.

In an embodiment, stimulator 16 may be integrated with a medical device,such as scalpel 470 shown in FIG. 14. Other medical devices that may beadapted to include a stimulator may be, for example, forceps, suctiondevices, scissors, needles, retractors, clamps, screws, or other similardevices. In an exemplary embodiment, the scalpel 470 may include anelectrode 480 that may be configured to provide a stimulus 34 to aportion of the subject. The electrode may be positioned in a locationthat may make first contact with the subject, such as the cutting edge490.

As generally illustrated in FIG. 15, the neural monitoring system 10 mayfurther include a transdermal stimulator 500 that may provide a stimulusto a portion of the subject 30 through a stimulator patch 502. In anembodiment, the transdermal stimulator 500 may provide an electricalstimulus to the subject 30 through the use of surface or needleelectrodes. In an exemplary use, a transdermal stimulator 500 may bepositioned on the subject's scalp to stimulate the motor cortex in atranscranial fashion. By stimulating the motor cortex, the motorpathways of the pyramidal tracts may be excited, which may be sensed asa mechanical or electrical response within the subject's muscles. Such atechnique may monitor motor evoked potentials (tcMEP) to evaluate theintegrity of the subject's neural pathways, such as during proceduresthat may put the spinal column at risk. The transdermal stimulator 500may be configured to deliver a transcranial stimulus on periodic basis;and, if an response is not detected by the one or more sensor devices 32after the delivery of the stimulus, the receiver 12 may be configured toprovide an alert to the user.

In another exemplary use, a transdermal stimulator 500 may be positionedon an extremity of a subject, and a sensing device may be positioned onthe subject's scalp. Stimulating the extremity may evoke a somatosensorypotential (SSEP) in the scalp that may be detected through an electricalsensor 102, and used to further evaluate the integrity of the subject'sneural pathways. If a somatosensory potential is not sensed by a sensingdevice 32 after the generation of the stimulus, the receiver 12 may beconfigured to provide an alert to the user.

In an embodiment, the transdermal stimulator 500 may be a stand-alonestimulator patch, or may alternatively be integrated with the sensingdevice 32 to provide a stimulus through electrodes 108, 110 (asgenerally illustrated in FIGS. 6-7). If the transdermal stimulator 500is integrated with the sensing device 32, tcMEP and SSEP responses maybe intermittently tested without a need to reconfigure the neuralmonitoring system 10.

The preceding description has been presented only to illustrate anddescribe exemplary embodiments of the methods and systems of the presentinvention. It is not intended to be exhaustive or to limit the inventionto any precise form disclosed. It will be understood by those skilled inthe art that various changes may be made and equivalents may besubstituted for elements thereof without departing from the scope of theinvention. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the inventionwithout departing from the essential scope. Therefore, it is intendedthat the invention not be limited to the particular embodiment disclosedas the best mode contemplated for carrying out this invention, but thatthe invention will include all embodiments falling within the scope ofthe claims. The invention may be practiced otherwise than isspecifically explained and illustrated without departing from its spiritor scope. The scope of the invention is limited solely by the followingclaims.

1. A method of detecting an induced muscle response comprising:monitoring a mechanomyographic (MMG) response of a muscle using amechanical sensor in mechanical communication with the muscle, the MMGresponse being attributable to a depolarization of a nerve innervatingthe muscle, the depolarization being in response to a providedelectrical stimulus; computing a time derivative of acceleration fromthe monitored MMG response; comparing the computed time derivative ofacceleration to a threshold; and identifying the occurrence of aninduced muscle response if the computed time derivative of accelerationexceeds the threshold.
 2. The method of claim 1, wherein monitoring theMMG response of the muscle comprises receiving a signal from themechanical sensor, the signal corresponding to a sensed acceleration. 3.The method of claim 2, further comprising: comparing the sensedacceleration to an acceleration threshold; and providing an alert if aninduced muscle response is detected and the sensed acceleration exceedsthe acceleration threshold.
 4. The method of claim 3, wherein theacceleration threshold includes a recurring event threshold and asingular event threshold; and wherein the recurring event threshold islower than the singular event threshold.
 5. The method of claim 1,further comprising receiving an indication of an amplitude of theprovided electrical stimulus, wherein the electrical stimulus isprovided by an invasive stimulator; and determining a distance betweenthe invasive stimulator and the nerve using the amplitude of theprovided electrical stimulus and a magnitude of acceleration of the MMGresponse.
 6. The method of claim 3, wherein the alert includes a visualalert.
 7. The method of claim 3, wherein the alert includes an audiblealert.
 8. The method of claim 2, wherein the threshold is an increasingfunction of the sensed peak acceleration.
 9. The method of claim 1,further comprising correlating the occurrence of an induced muscleresponse with the application of a stimulus.
 10. The method of claim 1,further comprising monitoring an electrical parameter of the muscleusing a plurality of electrodes in electrical communication with themuscle.
 11. A method of detecting an induced muscle response using asensing device comprising: providing an electrical stimulus; monitoringan acceleration from a mechanical sensor associated with the sensingdevice; computing a time derivative of acceleration from the monitoredacceleration; comparing the computed time derivative of acceleration toa threshold; identifying the occurrence of an induced mechanomyographic(MMG) muscle response if the computed time derivative of accelerationexceeds the threshold, and wherein the induced muscle response is inresponse to the provided electrical stimulus.
 12. The method of claim11, further comprising electrically determining if the sensing device isin contact with the subject.
 13. The method of claim 12, wherein thestep of electrically determining contact includes: monitoring acapacitance between a plurality of electrodes disposed on the sensingdevice; and comparing the monitored capacitance to a threshold.
 14. Themethod of claim 12, wherein the step of electrically determining contactincludes: monitoring an electric field between a plurality of electrodesdisposed on the sensing device; and comparing the monitored electricfield to a threshold.
 15. The method of claim 12, wherein the step ofelectrically determining contact includes: monitoring a respectivevoltage between a plurality of electrodes disposed on the sensingdevice; and comparing the voltage to a threshold.
 16. The method ofclaim 12, further comprising providing an alert if the sensing device isnot in contact with the subject.
 17. The method of claim 11, furthercomprising: comparing the sensed acceleration to an accelerationthreshold; and providing an alert if an induced muscle response isdetected and the sensed acceleration exceeds the acceleration threshold.18. The method of claim 17, wherein the acceleration threshold includesa recurring event threshold and a singular event threshold; and whereinthe recurring event threshold is lower than the singular eventthreshold.
 19. The method of claim 11, further comprising: receiving anindication of the amplitude of the provided electrical stimulus, whereinthe electrical stimulus is provided by an invasive stimulator; anddetermining a distance between the invasive stimulator and a nerveinnervating the muscle using the amplitude of the provided electricalstimulus and a magnitude of the monitored acceleration.
 20. The methodof claim 11, wherein the threshold is an increasing function of thesensed peak acceleration.
 21. The method of claim 11, further comprisingcorrelating the occurrence of the induced MMG muscle response with theapplication of a stimulus.